Surface lighting devices have been used for backlighting of liquid crystal displays (LCD) employed in cellular-phones, portable terminals and the like. Light emitting diodes (LED) in chip-shape have been used as light sources of this surface lighting device because of LED's small size and low power consumption. Lately, LEDs replace cold-cathode tubes as surface lighting devices of various portable-terminals including information terminals, digital cameras, camcorders because LEDs are superior in smaller size, longer battery life, and withstanding shocks.
Various structures are available in those conventional surface-lighting-devices using LEDs. For instance, Japanese Patent Examined Application Publication No. H03-32075 discloses that LEDs are disposed behind an LCD panel thereby emitting light directly to the back face of the LCD panel in order to light the panel. However, thinner bodies of the terminals are required in the market, and problems of disposing electronic circuits behind LCD elements are not favorable for this structure, i.e. a structure where LEDs are disposed directly behind the LCD panel becomes inconvenient. Therefore, fewer models of cellular phones and the like use this structure as the surface lighting device for LCD.
Regarding the surface lighting device for LCDs of cellular phones, LEDs are disposed outside the display face of LCD elements in many cases. For instance, Japanese Patent Examined Application Publication No. H05-21233 teaches that LEDs are disposed outside the display face of LCD elements so that the light of the LEDs is guided under the LCD elements by using a reflective face and a resin board.
However, the conventional surface lighting device as discussed above distributes uneven luminance, i.e. the light is distributed not uniformly if only one LED is provided at the center of the device. In this case, neighbor of the LED is only well-lighted, and periphery of the LED is poorly-lighted. Such luminance distribution becomes more obvious at a greater area to be lighted. This uneven luminance distribution would result in poor readability of the display, and further, produce an unrecognizable displayed section due to poor-light. To overcome this problem, a number of LEDs as a light source is increased, and spaces between the LEDs arrayed are narrowed, so that the luminance distribution is improved. This is a conventional measure against the problem.
When the number of LEDs is increased in the surface lighting device of the LCD used in cellular phones and other portable terminals, it not only boosts the power consumption, but also incurs complicated works for mounting the LEDs as well as a cost increase.
When a plurality of LEDs are used, differences between wavelengths of lights emitted from the LEDs produce an uneven color. To be more specific, in many cases, each LED differs in a light wavelength emitted therefrom by several nano-meters due to individual characteristics. Such a little difference; however, produces great unevenness in a color for human eyes when respective areas illuminated by each LED and their borders are compared each other. In order to eliminate the unevenness in a color, emit each LED and measure the light wavelength, thereby collecting the LEDs having the same wavelength. This operation is required to assemble one surface lighting device free from a dispersion of the light wavelengths. This operation; however, requires a cumbersome 100% inspection on the LEDs, and this is one of the causes to lower the productivity of the surface lighting devices.
Also, luminance differences among the LEDs due to individual characteristics produce uneven luminance of the surface lighting device.
Another method for improving the unevenness in a color and luminance is to use one LED, and a light guiding section of a light guide board is elongated so that light can be scattered sufficiently. Then the light is emitted from a light emitting section. This method; however, limits the downsizing of the surface lighting devices.